The present invention relates to a push-pull amplifier, and more particularly to an amplifier circuit which makes it possible to automatically adjust a biasing current.
FIG. 1 is a circuit diagram of a conventional push-pull amplifier.
Referring to FIG. 1, Q.sub.1 and Q.sub.2 denote NPN and PNP transistors, respectively, R.sub.1 and R.sub.2 resistors for stabilizing a biasing current I.sub.B, Z.sub.L load including reactance, V.sub.BS /2 biasing power supply circuit, and +V.sub.CC and -V.sub.CC DC voltage power supplies, respectively.
If the input voltage e.sub.i is 0 volts, the electric potential at the output terminal P is approximately 0 volts. Since no load current I.sub.L flows, only a biasing current I.sub.B .perspectiveto.[V.sub.BS -(V.sub.BE1 +V.sub.BE2)]/(R.sub.1 +R.sub.2) flows between transistors Q.sub.1 and Q.sub.2.
However, assuming that the input voltage e.sub.i varies towards positive-going swing, the load current I.sub.L flows in the direction of the solid line shown in FIG. 1 from the transistor Q.sub.1. Due to the voltage drop I.sub.L R.sub.1 across the resistor R.sub.1, the biasing current I.sub.B decreases.
If the input voltage e.sub.i further increases and I.sub.L R.sub.1 .gtoreq.V.sub.BS -(V.sub.BE1 +V.sub.BE2) holds, the transistor Q.sub.2 will be completely cut off.
In this condition, the push-pull circuit becomes equal to an emitter-follower circuit in which only transistor Q.sub.1 is used, resulting in deterioration in the transient characteristics if the load Z.sub.L includes reactance.
On the other hand, when the input voltage e.sub.i varies towards negative-going swing, the load current I'.sub.L flows in the direction of the broken line in FIG. 1 to the transistor Q.sub.2.
Accordingly, when I'.sub.L R.sub.2 .gtoreq.V.sub.BS -(V.sub.BE1 +V.sub.BE2) holds, transistor Q.sub.1 is completely cut off, resulting in the same phenomenon.
In order to prevent this the push-pull circuit may, for instance, be greately biased to effect class "A" amplification. Thus, if the circuit is designed so that a biasing current I.sub.B flows, serving as an idling current whose value is of the order of magnitude of the load current I.sub.L, some improvement may be expected.
However, with this conventional circuit, a low efficiency of amplification will be obtained. Moreover, in order not to allow transistors Q.sub.1 and Q.sub.2 to be completely cut off, the minimum impedance value of the load is limited to a predetermined value. Therefore, this method is not very satisfactory.